This invention relates to the field of tooling manufacture by layered fabrication techniques and more particularly to three dimensional printing of metal and metal/ceramic molds.
Metal molds for forming processes such as injection molding, blow molding, die casting, forging, and sheet metal forming are currently made using manufacturing techniques such as machining, EDM, casting and electroforming. (K. Stoeckhert (ed.), xe2x80x9cMold Making Handbook for the Plastics Engineer,xe2x80x9d Oxford University Press, New York, N.Y. 1983.) The creation of the tool is a multi-step process involving a variety of manufacturing techniques. The mold is created by removing material from a block of metal, usually a tool material such as tool steel. Typically, a block of annealed tool steel is first rough machined to near-net shape. The near-net shape tool may then be hardened with an appropriate heat treatment cycle to obtain the desired final material properties. Final dimensions are obtained by finish machining, grinding or EDM of the mold pieces. Final finishing may also occur before the hardening step. Selected tool surfaces are then modified as required. Mating surfaces are typically ground to provide adequate sealing. Surfaces which require additional hardness or abrasion resistance can be treated by techniques such as nitriding, boriding, plating or ion implantation. (K. Stoeckhert (ed.), xe2x80x9cMold Making Handbook for the Plastics Engineer,xe2x80x9d Oxford University Press, New York, N.Y. 1983.)
Alternate techniques exist for creating the near-net shape tool pieces, such as casting or electroplating. Near-net shape tool ingots made by casting are produced using established casting techniques. After casting, the metal perform must be finished using the additional finish machining or EDM processes described above. Tool performs made by electroforming are produced by electroplating a metal, typically nickel, onto an appropriately shaped mandrel. After plating to sufficient thickness, the tool is removed from the mandrel. Although the face of the tool is completely defined by the electroforming process, other portions of the tool must be created using other processes. A backing material, such as metal-filled epoxy, must be added to the rear of the tool and machined to the appropriate shape before the tool can be used. An alternate method of producing metal tools, the Tartan Tooling method, is described in U.S. Pat. No. 4,431,449 and U.S. Pat. No. 4,455,354. In this method, metal powder is packed around a negative of the shape to be produced and bonded with a polymeric material. The negative can be produced by any convenient means. The bonded powder green part is then fired to remove the polymer and to partially sinter the part. Finally, the porous sintered part is infiltrated with a lower melting point alloy to fill the residual porosity, producing a fully dense metal tool. Tools produced by the Tartan Tooling method have fewer finishing requirements than near-net shape performs made by other processes, but some finishing is usually required.
Metal molds, tools and dies produced by the above techniques must meet a variety of performance requirements. These requirements are determined by the type of forming processes the mold will be used for. Tools used for injection molding, for example, must remove heat from the injected part to cool it and freeze its shape. The transfer of heat away from the molten plastic directly affects part cycle time, dimensional accuracy and material properties. (K.Stoeckhert (ed.), xe2x80x9cMold Making Handbook for the Plastics Engineer,xe2x80x9d Oxford University Press, New York, N.Y. 1983.) Molds with poor heat transfer characteristics require longer waiting periods before the plastic part has solidified enough to be ejected without damage, thus increasing cycle time. Molds in which the polymer freezes non-uniformly due to uneven heat removal can result in anisotropic shrinkages across the part, causing part warpage and loss of dimensional control. Additionally, residual stresses are incorporated into the plastic part during uneven cooling, having a detrimental effect on the material properties of the part. Injection molds typically incorporate fluid coolant channels to increase the rate at which heat can be removed from the injected plastic. (R. G. W. Pye, xe2x80x9cInjection Mould Design,xe2x80x9d 4th ed., Longman Scientific and Technical, Essex, England, 1989.) The coolant channels are incorporated into the mold using traditional machining or EDM techniques. The layout of the coolant channels is dependent on part geometry and the specific limitations of the processes used to create the channels.
Cooling of a tool can be effected in one of two ways. For smaller parts in which a tooling insert mold 1 is used, as shown in FIG. 1, the coolant channels 2 are located in the backing plate assembly 3, which also provides the majority of the mechanical support necessary to resist mold deflection during the injection cycle. (H. Gastrow, xe2x80x9cInjection Molds: 102 Proven Designs,xe2x80x9d Hanser Publishers, Munich, 1983.) Channels are not directly incorporated into the insert itself because of the additional expense and fabrication time. Also, the size of channels for insert molds may be prohibitively small, making fabrication difficult. For larger, non-insert type molds 4, shown in FIG. 2, the coolant channels 5 are directly incorporated into the mold. In both cases, the location and configuration of the coolant channel layout is a compromise between ideal cooling and practical limitations. For insert molds, the backing plate coolant channel layout is not tailored to a specific insert mold but is designed with a generic layout, and therefore cannot meet the ideal coolant needs of a particular insert geometry. Additionally, the heat flux transferred from the mold insert to the coolant plate must pass through the gap between the insert and plate surfaces. Although this gap is usually filled with a heat conductive grease or other material, the overall heat transfer is lessened. For larger molds 9, shown in FIG. 3, the coolant channels are usually arrays of straight cylindrical holes 10 which are made using standard machining procedures. The channels are incorporated into the mold as an additional mold fabrication step after the mold cavity or core has been defined. The actual layout of the channels is limited by the shape of the cavity or core, in addition to fabrication constraints. The simple straight cylindrical channels cannot follow the complex contours of a typical cavity or core, resulting in uneven cooling of the mold surfaces. Also, the number and placement of the channels cannot be allowed to compromise the mechanical integrity of the mold. Also, rework of coolant channels, as might be required if the initial configuration does not perform adequately, becomes increasingly difficult as more mold material is removed. Improper layout of the channels may require the entire mold to be scraped.
The cylindrical holes are usually created by drilling. The cylindrical shape of the coolant channel is therefore a consequence of the manufacturing technique used to create it and not because it is the ideal shape for heat transfer purposes. The internal surface area of the channel, which directly effects the overall heat transfer from mold metal to coolant, is limited by this requisite cylindrical shape. Increasing channel diameter or the number of channels in the mold are ways to increase the effective channel wall surface area, but these techniques are limited by mold geometry. Other methods of increasing heat transfer which are commonly found in heat exchanger design, such as finned or textured surfaces, are not readily adaptable to mold coolant channels made by conventional means.
Most tool steel alloys are tailored for high strength, hardness and toughness in order to survive millions of injection cycles. Tool steels typically have fair to poor thermal conductivity values compared to other softer tooling alloys, specifically copper alloys, although the copper alloys cannot match the strength and hardness of tool steel. Since the components of injection molding tools are made almost exclusively from a single material, a compromise has to be made with regard to either high strength or high thermal conductivity properties. In some very complicated, multi-component tools, different parts of the tool can be made from different alloys. For example, a tool steel mold core 15 with a high aspect ratio can be drilled out and a dowel pin 16 made from high thermal conductivity metal can be inserted to provide an enhanced heat transfer path to an adjacent coolant channel 17, as shown in FIG. 4. Tool modifications of this type can only be used in a limited set of geometries and add to the complexity of fabrication and cost of the tool.
Most injection molds are actively cooled so that heat will be removed quickly from the injected plastic, allowing for faster plastic solidification and decreased cycle times. The coolant fluid supplied to the mold is at a preset temperature. Regardless of the temperature chosen, however, a compromise invariably results between finished part quality and cycle time. If a high coolant temperature is chosen, the finished part will have low internal stress and good dimensional accuracy. Cycle time, however, is lengthened due to the slow cooling. If a low coolant temperature is chosen, the cycle time is reduced due to faster cooling, but the rapid and potentially non-uniform freezing results in parts of lesser quality. A method for simultaneously achieving low cycle time and high part quality would involve rapidly heating the mold by electrical resistance just before injection, and then rapidly cooling the mold after injection to decrease cycle time. (B. H. Kim, xe2x80x9cLow Thermal Inertia Injection Molding,xe2x80x9d MIT Ph.D. Thesis, 1983.)
Metal molds for forming processes can also be created directly from a computer model using processes that construct objects in layers, such as the three dimensional printing process described for example in U.S. Pat. Nos. 5,204,055, 5,340,656, and 5,387,380. In a typical application of this process, the mold is created by spreading a powder layer using a roller within a confined region as defined by a piston and cylinder arrangement. A water soluble polymer is then deposited at specific regions of the layer as determined by a computer model of the mold. The water soluble polymer acts to bind the powder within the layer and between layers. This process is repeated layer after layer until all layers needed to define the mold have been printed. The result is a bed of powder which contains within it a polymer/metal composite of the desired geometry. Unbound powder temporarily supports unconnected portions of the component, allowing overhangs, undercuts and internal volumes to be created. Next, the entire bed is heated to a temperature sufficient to cure the polymer and bind the printed regions together. The mold is then removed from the loose, unprinted powder to provide a green metal mold. A variety of post-processing options exist for transforming the three dimensional printed green metal mold into a fully dense, all metal part. The polymeric binder must first be removed by thermal decomposition in a process called debinding. One method of debinding is accomplished by firing the part in a non-oxidizing atmosphere at temperatures in excess of 500xc2x0 C. (R. M. German, xe2x80x9cPowder Injection Molding,xe2x80x9d Metal Powder Industries Federation, Princeton, N.J., 1990.) After debinding, additional processing is performed to fully density the part. For example, the debound part is directly sintered to full density using a heating schedule appropriate to obtain full densification by sintering. In this approach, debinding and sintering can occur in a single continuous operation. Another means of obtaining full density is to sinter the metal skeleton after debinding such that only part of the porosity is eliminated. Again, debinding and sintering can occur in a single continuous operation. In a second firing operation, the metal skeleton is infiltrated with a Lower melting point alloy, thereby filling the residual porosity with infiltrant metal. The amount of sintering required before infiltration will depend on the infiltration alloy and infiltration temperature. The sintered metal skeleton must be strong enough to resist the capillary forces induced by the liquid infiltrant. Typically, sintered densities in excess of 65% of theoretical are sufficient. Typically, the green part would be printed at about 60% theoretical density and sintered to 65% theoretical density.
The current invention provides tooling, particularly metal and ceramic/metal molds, produced by solid free form fabrication techniques with unique thermal features and properties. The invention is particularly suited for use with three dimensional printing techniques.
According to one aspect of the invention, molds are made with internal contour cooling channels which are incorporated into the mold during the printing process. There are no restrictions on the number, size, shape and routing of these channels. The internal channels can be printed so as to be completely open, or they can be filled with a porous material. The shape and texture of the cooling channel walls can be optimized for high heat transfer by increasing channel surface area and modifying channel surface texture. For example, the channels can incorporate thermal structures such as fins to increase surface area to enhance heat transfer. The surface texture of the internal channels can be controlled by the selection of the powder and binder used to print the green part.
During fabrication of the mold, the cooling channels can be left open during the infiltration step by creating a pressure differential between ambient and the infiltrant melt front, such as by placing the mold on stilts of a predetermined minimum height through which the infiltrant rises by capillary action. Also, by printing an xe2x80x9cinfiltration stopxe2x80x9d material in appropriate regions of the tool volume, the cooling channels can be left open after the infiltration step of post-processing.
In another aspect of the invention, internal cellular structures, preferably in combination with cooling channels, are created beneath the mold surface during printing. The thermal inertia of the mold is reduced by such a cellular structure, allowing for rapid thermal cycling of the mold.
The invention also provides enhanced thermal properties created by the selective deposition of high thermal conductivity materials, thereby increasing heat transfer from the tool surface to the coolant channels. For example, high thermal conductivity paths can be created between the mold surfaces and the coolant channels. Selective printing of high thermal conductivity materials additionally allows the creation of gradual compositional gradients within the tool, thereby minimizing local stresses caused by the local variation in coefficient of thermal expansion and increasing the overall toughness of the tool.
Buried pockets of a high thermal conductivity material can also be provided by incorporating a pocket open on one end during the printing process, removing unbound powder from the pocket through the open end, sealing the open end, and infiltrating the part with an infiltrant of a high thermal conductivity material to fill the pocket.